(1) Field of the Invention
The invention relates to a process for the production of a low-sulfur diesel gas oil having a low-sulfur content and a good color from a petroleum distillate. More particularly, the invention relates to a process for the production of a low-sulfur diesel gas oil having a sulfur content of 0.05% by weight or lower and having a Saybolt color number of -10 or higher, from a petroleum distillate having a sulfur content of 0.1 to 2.0% by weight and having an inferior color and inferior oxidation stability.
(2) Description of the Prior Art
At present, domestic diesel gas oils are produced by blending a desulfurized gas oil fraction obtained by conventional desulfurization of a straight-run gas oil with a straight-run gas oil fraction, a straight-run kerosine fraction, a gas oil fraction obtained by petroleum cracking, or the like to thereby provide a product having a sulfur content of 0.4 to 0.5% by weight.
Recently, due to rising concern regarding environmental problems, further reduction of NO.sub.x and particulate matter evacuated with exhaust fumes from diesels is required.
The basic requirements requested to the petroleum manufacturers are as follows:
1. First, the sulfur content of 0.4 to 0.5% by weight as set forth in the present standards must be lowered to the first-phase targeted exhaust fume standard of 0.2% by weight.
2. The sulfur content of 0.2% by weight must be further lowered to the second-phase targeted standard of 0.05% by weight in stages.
3. As for the color, no targeted standards have been fixed yet; however, each oil refining company has been carrying out quality control for diesel oils by setting a reference standard for color in various scales such as Saybolt, ASTM, APHA color numbers, or the like from an independent standpoint.
Especially, cracked gas oils used as a base gas oil, for which a large increase in demand in the future has been forecasted, are very poor in color; therefore, a distinct improvement on the present color level is also required. Japanese Patent Laid-Open Application No. 3-86793 proposes a two-step hydro-treating process for the production of a diesel gas oil having a sulfur content of 0.2% by weight or lower which is the first-phase targeted standard. However, under the conditions of the process such as a pressure of 10 to 40 kg/cm.sup.2, a temperature of 280.degree. to 370.degree. C., a liquid hourly space velocity, LHSV, of 0.5 to 5.0 hr.sup.-1 in the first step, and a pressure of 10 to 40 kg/cm.sup.2, a temperature of 150.degree. to 325.degree. C., an LHSV of 0.5 to 5.0 hr.sup.-1 in the second step, it is extremely difficult to meet the second-phase targeted standard of sulfur content of 0.05% by weight.
Further, under a pressure of 40 kg/cm.sup.2 or lower in the second reactor in this case, it is extremely difficult to meet the requirement for color i.e. the reference standard for the color of finished products, because the desulfurized oil to be fed into the second step has already been treated at a much higher temperature in the first step in order to meet a sulfur content of 0.05% by weight and thus has a poor color. Such difficulty is encountered especially remarkably in hydro-treating of a cracked gas oil which is not good in color.
U.S. Pat. No. 4,755,280 teaches a two-step hydro-treating process for improving the color or oxidation stability of hydrocarbon compounds wherein an Fe-type catalyst is employed in the second reactor in order to improve the color and oxidation stability. However, it is known that the hydro-treating activity of the Fe-type catalyst is readily poisoned with hydrogen sulfide and the like (Japanese Patent Laid-Open Application No. 62-84182). Therefore, the amounts of sulfur and nitrogen compounds such as hydrogen sulfide and ammonium present in the feedstock to be fed to the second step must be lowered to a total amount of about 10 ppm or lower prior to feeding.
As it is seen from the description of the above process, when sulfur and nitrogen compounds such as hydrogen sulfide and ammonium present in the materials issued from the first step must be removed prior to feeding the materials into the second step, it is necessary to install additional units such as a vapor-liquid separator, a stripper for the stripping of the absorbed hydrogen sulfide and ammonium from the rich oil, and a washing tower for the removal of these compounds present in the rich gas; therefore, a commercial plant of this type is very costly, and increases the costs of operation unpreferably.
U.S. Pat. No. 3,841,995 proposes a two-step hydro-treating process for the improvement of the color and odor of hydrocarbon compounds. However, in the process a noble metal catalyst such as Pt is employed in the second reactor; thus, the hydro-refining activity of the catalyst is readily poisoned by hydrogen sulfide and the like. Therefore, it is necessary to remove the sulfur and nitrogen compounds such as hydrogen sulfide and ammonium present in the materials issued from the first step to thereby provide a hydrogen sulfide- and ammonium-free feedstock to be fed to the second step. This is costly in a similar manner as described for the process disclosed by U.S. Pat. No. 4,755,280.